The present invention relates to a coin-shaped battery having a flat cylindrical shape, and to a coin-shaped battery having a case structure with improved liquid leakage resistance under the circumstances where an excessive thermal stress is applied to the battery, such as under a high temperature.
Coin-shaped batteries, which are also referred to as button-shaped batteries or flat batteries, are widely used for their small and thin size, in equipment required to have reduced size such as watches and remote automotive entry systems and in the case where long-time use is required such as for memory backup of OA equipment or FA equipment. In addition, the coin-shaped batteries are sometimes used as power sources for various meters under a high temperature.
FIG. 5 is a cross-sectional view showing a coin-shaped battery having a conventional structure. This coin-shaped battery includes a cathode tablet 33 and an anode tablet 32, which are placed in a cathode case 31 so as to be opposed to each other with a separator 34 interposed therebetween. After the cathode case 31 is filled with an electrolyte, an opening of the cathode case 31 is crimped with an anode cap 35 through a gasket 36 interposed therebetween, thereby completing the coin-shaped battery. In the coin-shaped battery having such a conventional structure, liquid leakage sometimes occurs due to continuous use under a high temperature and/or due to a sudden change in temperature. The reason for occurrence of such liquid leakage is as follows. A rise in temperature induces the expansion and/or vaporization of an electrolyte, or the like. When an internal pressure of the battery rises, the anode cap 35 and the cathode case 31, from a normal state shown in FIG. 6, outwardly bulge as in a state shown in FIG. 7. At this point, the force for pushing the gasket 36 up acts on an end of the anode cap 35 to separate the anode cap 35 from the gasket 36. Moreover, a gap is generated between the cathode case 31 and the gasket 36 due to outward bulge of the cathode case 31. Such deformation of a crimped portion is likely to cause the liquid leakage.
In order to prevent such liquid leakage, countermeasures as follows have been implemented: reduction in the amount of an electrolyte filling the cathode case, increase in thickness of the cathode case 31 and the anode cap 35 for preventing the deformation, and the like. In addition, there has been known a method of restraining the deformation of a crimped portion of a cathode case 41 upon rise in internal pressure of a battery, with an indent structure as shown in FIG. 8.
However, with reduction in the amount of the electrolyte filling the cathode case, an energy density per volume of the battery is naturally decreased. When the thicknesses of the cathode case 31 and the anode cap 35 are increased, an internal volume of the battery is reduced, also resulting in reduction in energy density per volume.
The indent structure has a disadvantage in that the force for compressing a gasket 46, which is applied to an anode cap 45 on sealing of an opening, induces the positional offset and the deformation of a washer 48 if an indent width is insufficient. As a result, a bottom of the gasket 46 is not sufficiently compressed. What is worse, the indent structure has another disadvantage in that a shoulder portion of the gasket 46 is not sufficiently compressed because the positional offset and the deformation of the washer 48 cause the displacement of the gasket 46 and the anode cap 45 in an inner diameter direction of the cathode case 41. The positional offset and the deformation of the washer 48 can be restrained by processing the indent width so as to protrude at least up to the center of a turn-up portion of the anode cap 45. With current press techniques, however, it is difficult to stably form a sufficient indent width. The unevenness in indent width thus results in ununiform leakage resistance of the battery.
The present invention has an object of providing a coin-shaped battery having a sealing structure that prevents liquid leakage even upon rise in internal pressure of the battery.
In order to achieve the above object, a coin-shaped battery according to the present invention includes: a cathode case formed so as to have a cylindrical shape with a closed end; and power generating elements housed within the cathode case, the cathode case and an anode cap being crimped with a gasket interposed therebetween, wherein the anode cap having a reversed cylindrical shape with a closed end with respect to the cathode case, includes a flange in its opening, the flange having, on its tip, an extended portion in a cylindrical direction and a turn-up portion upwardly extending from a bent portion corresponding to the lowermost end portion of the extended portion; the cathode case includes, in a periphery of its bottom face, a rising portion having an outer diameter smaller than an inner diameter of the extended portion, and a crimp portion further extending from the rising portion; and the gasket is placed so that its upper portion is compressed between an upper end of the crimp portion, and a tip of the turn-up portion and the flange, and its lower portion is compressed between a step portion of the crimp portion and the bent portion, when the crimping is effectuated.
According to this configuration, when the cathode case bulges due to rise in internal pressure of the battery, a deformation pressure is dispersed by the step portion. When the anode cap bulges, a deformation pressure is dispersed by the flange. Consequently, the deformation of the crimped portion is restrained. The thus sealed battery demonstrates excellent liquid leakage resistance even on the use under a high temperature or the like.
In the above configuration, the rising portion from the bottom face of the cathode case to the step portion is formed upwardly from the bottom face, so that the deformation of the bottom face of the cathode case is restrained from propagating to the crimped portion. Thus, the rising portion is effective to maintain the liquid leakage resistance.
When the step portion is formed so as to have a width in a radius direction of 1.5 mm to 2.5 mm and a height from the bottom face of the cathode case of 0.5 mm to 1.5 mm, it is suitable for restraining the deformation of the cathode case from propagating to the crimped portion.
A height of the tip of the turn-up portion is formed so as to be identical with that of an outer face of the flange or to fall within the range of a thickness of the anode cap. Thereby, the degree of compression of the gasket by the flange and the tip becomes uniform. Thus, when the deformation propagates to the crimped portion, the deformation of the anode cap is restrained, thereby maintaining the functions for preventing the liquid leakage.
Furthermore, in order to restrain the occurrence of liquid leakage due to rise in internal pressure of the battery, the coin-shaped battery of the invention includes a plurality of parts for compressing the gasket so that an excessive compression is not locally applied to the gasket. More specifically, the gasket is compressed by the flange of the anode cap and the tip of the turn-up portion. Even upon rise in internal pressure, excessive compression does not occur at any site. As a result, the rupture and/or breakage of the gasket caused along with excessive compression of the gasket is restrained to prevent the liquid leakage from occurring.
The coin-shaped battery of the invention is preferred to have a structure with enhanced elastic deformability of the gasket. In this configuration, even when a high compressive force is applied to the gasket, the gasket is capable of following the deformation of the anode cap and the cathode case because the gasket can restore its steady state owing to elastic deformation. Thus, the generation of a gap between the gasket and the anode cap/the cathode case is restrained. As a material of the gasket with enhanced elastic deformability, an elastoplastic resin having a tensile yield strength of 40 MPa or higher, or an elastic resin made of a rubber-type elastomer is extremely suitable. The use of these resin materials allows the minimization of the effects of deformation on the sealing performance even when a thermal stress applied under severe temperature circumstances, in particular, a rise in internal pressure of the battery, induces the deformation of the anode cap and the cathode case. Consequently, the liquid leakage is effectively restrained from occurring.